In recent years, OCT that forms images of the surface morphology and internal morphology of an object by using a light beam from a laser light source or the like has attracted attention. Unlike an X-ray CT apparatus, optical coherence tomography is noninvasive to human bodies, and is therefore expected to be utilized in the medical field and biological field. For example, in the ophthalmology, apparatuses that form images of a fundus and cornea or the like are in a practical stage.
The apparatus disclosed in Patent Document 1 uses a technique of so-called “Fourier Domain OCT.” That is to say, the apparatus irradiates a low-coherence light beam to an object, superposes the reflected light and the reference light to generate an interference light, and acquires the spectral intensity distribution of the interference light to execute Fourier transform, thereby imaging the morphology in the depth direction (the z-direction) of the object. Furthermore, the apparatus is provided with a galvano mirror that scans a light beam (signal light) along one direction (x-direction) perpendicular to the z-direction, and is thereby configured to form an image of a desired measurement target region of the object. An image formed by this apparatus is a two-dimensional cross sectional image in the depth direction (z-direction) along the scanning direction (x-direction) of the light beam. The technique of this type is also called Spectral Domain.
Patent Document 2 discloses a technique of scanning with a signal light in the horizontal direction (x-direction) and the vertical direction (y-direction) to form multiple two-dimensional cross sectional images in the horizontal direction, and acquiring and imaging three-dimensional cross sectional information of a measured range based on the cross sectional images. As the three-dimensional imaging, for example, a method of arranging and displaying multiple cross sectional images in the vertical direction (referred to as stack data or the like), or a method of executing a rendering process on volume data (voxel data) based on stack data to form a three-dimensional image may be considered.
Patent Documents 3 and 4 disclose other types of OCT apparatuses. Patent Document 3 describes an OCT apparatus that images the morphology of an object by sweeping the wavelength of light that is irradiated to an object (wavelength sweeping), detecting interference light obtained by superposing the reflected lights of the light of the respective wavelengths on the reference light to acquire its spectral intensity distribution, and executing Fourier transform. Such an OCT apparatus is called a Swept Source type or the like. The Swept Source type is a kind of the Fourier Domain type.
Further, Patent Document 4 describes an OCT device that irradiates a light having a predetermined beam diameter to an object and analyzes the components of an interference light obtained by superposing the reflected light and the reference light, thereby forming an image of the object in a cross-section orthogonal to the travelling direction of the light. Such an OCT device is called a full-field type, en-face type or the like.
Patent Document 5 discloses an example of applying OCT to the ophthalmologic field. It should be noted that, before OCT was applied, a retinal camera, a slit lamp microscope, etc. were used as apparatuses for observing an eye (see Patent Documents 6 and 7, for example). The retinal camera is an apparatus that photographs the fundus by projecting illumination light onto the eye and receiving the reflected light from the fundus. The slit lamp microscope is an apparatus that obtains an image of the cross-section of the cornea by cutting off the light section of the cornea using slit light.
The apparatus with OCT is superior relative to the retinal camera, etc. in that high-definition images can be obtained, further in that cross sectional images and three-dimensional images can be obtained, etc.
Thus, the apparatus using OCT can be used for observation of various regions of the eye and is capable of obtaining high-definition images, and therefore, has been applied to the diagnosis of various ophthalmic disorders.
Now, there are three important viewpoints in ophthalmological practice. They are “early detection”, “center management” and “progression management”. Although early detection and progression management are thought to be important in other medical department, center management is specific to ophthalmology. There is a site called central fovea in an eye fundus. The central fovea is located at the center of the macula area of a retina, contributes to eyesight at the central visual field with high resolution, and is a most important region in the eye fundus. Thus, it is judged whether or not a disease exists in a near-field of the central fovea, and treatment plan is determined according to the judgment. This is center management. For example, medical diagnosis and treatment for glaucoma is carried out by especially focusing on the state of the area within 5 degrees around central fovea. A perimeter is used for the examination for this purpose (see Patent Document 8, for example).
Further, it is also carried out to acquire information of an eye fundus (in particular, thickness of a layer in an eye fundus) by using OCT (see Patent Documents 9, 10 and 11, for example).